Road mills, sometimes called cold planers or scarifiers, are work machines that typically include a frame quadrilaterally supported by tracked or wheeled support units. The frame supports machine components, including an engine, an operator's station, and a milling drum. The milling drum, fitted with a plurality of milling tools, is rotated through a suitable interface by the engine to break up a road surface.
The support units generally include lift columns mounted between the frame and the tracks or wheels. Extending or retracting the lift columns raises or lowers the frame and milling drum relative to the tracks or wheels and, consequently, relative to the ground. At least one of the support units, typically a rear unit, is commonly constructed in a manner permitting it to swing or pivot between two different operating positions: a projecting position in which the track or wheel is positioned substantially outside of the boundaries of the machine frame for maximum stability, and a retracted position in which the track or wheel is positioned substantially within the boundaries of the machine frame to enable the machine to mill road surfaces close to a curb or wall, for example.
Typically, the tracks or wheels, including the pivotable unit, are driven for traction purposes by individual hydraulic motors. The necessary pressurized hydraulic fluid is supplied by a pump driven by the frame mounted engine. To move the pivotable support unit from one position to the other position, an operator uses the lift column to lower the frame with respect to the support unit until the milling drum (or another frame mounted component) rests on the ground. Continued operation of the lift column raises the track or wheel off the ground so that the support unit can be pivoted. However, absent some correction mechanism, repositioning the support unit in this manner also causes the track or wheel to reverse its direction of rotation or running direction. Consequently, it is desirable to counter-rotate the track or wheel relative to the rotation caused by the repositioning to maintain the original alignment and direction of rotation, regardless of whether the support unit is in the projecting or retracted position.
EP 0 916 004 proposes using a guide-rod gear to provide a pivotable support unit with a counter-rotatable wheel. The guide-rod gear is shown connected between the machine frame and the support unit, and consists of a four-bar linkage mechanism having four vertical articulated axles and two guide rods pivotable in a horizontal plane. A single hydraulic actuator causes the four-bar or parallelogram type linkage to pivot the rear wheel supported by a non-rotatable lift column between the projecting and retracted positions, while counter-rotating the wheel and lift column. This design causes the weight of the machine resting on the pivotable rear wheel to be carried by the four-link mechanism, which may result in reduced stability and stiffness of the machine. Also, precise and potentially wear-prone couplings have to be employed.
Further, road mills must be steered, and optimum steering angles differ in accordance with the well-known Ackerman principle when the support unit is in the retracted position versus the projecting position. This is a particular problem when the machine is fitted with tracks instead of wheels, because the rear tracks, especially the retracted rear track, must be steered in concert with the front tracks to avoid dragging or skipping of the rear tracks on the road surface. The single actuator guide-rod system of EP 0 916 004 does not provide integrated steering capability of the pivotable rear wheel, and is not well suited for use with machines fitted with tracks.
International publication WO 02/103117 describes another road mill of the general construction discussed above, and offers improvements over the guide-rod system. Instead of a four-bar linkage, the support unit is mounted on a sturdy support or swing arm that is pivotally connected to the machine frame with a single large pivot pin. This arrangement eliminates the need for a multi-piece linkage, such as the guide rod gear, with numerous pivot joints. The support arm may be pivoted by means of a linear hydraulic cylinder connected between the arm and the frame. A second linear hydraulic cylinder is described connected between the support arm and an axially rotatable portion of the lift column that is, in turn, connected to the track or wheel. When the support arm is pivoted by the one hydraulic cylinder the track or wheel may be counter-rotated by the other hydraulic cylinder, allowing the support unit to swing between the projecting and retracted positions while maintaining constant the running direction of the associated track or wheel. Because of the independent action of the two hydraulic cylinders, steering of the pivotable track or wheel can be accomplished using the second hydraulic cylinder, making this design suitable for use with machines fitted with either tracks or wheels.
The above described mechanisms provide different solutions to the problem of pivoting a machine support unit between projecting and retracted positions while maintaining the running direction of the pivoted track or wheel, but both solutions place bulky mechanical devices at a location on the support unit which must fit into a tightly confined space, especially in the retracted position. In addition, the guide rod gear arrangement is not suitable for steering the support unit, and fine steering control can be difficult to achieve using the hydraulic linear cylinder arrangement. The present invention is directed to overcoming one or more of these and other problems or disadvantages associated with the prior art.